Big-headed turtle (Platysternon megacephalum)

The big-headed turtle is classified as Endangered (EN) on the IUCN Red List (1), and listed on Appendix II of CITES (3). Five subspecies are currently recognised (2).

The most distinctive feature of the aptly named big-headed turtle (Platysternon megacephalum) is its enormous, triangular head, which is so large in proportion to its body that it cannot be withdrawn into the shell for protection (4) (5). Instead, the top and sides of the turtle’s head are covered with a large bony ‘roof’ that acts as armour and, unlike most turtles, the skull is solid bone (4) (5). The upper jaw is hooked and the edges of both jaws are lined with another tough horny covering, leaving only a narrow band of unprotected skin on the cheeks and running from the turtle’s eyes to the corners of its mouth (4). Together with an oversized head, this odd-shaped turtle can be recognized by its exceptionally long, thin tail and particularly flattened shell, which is somewhat rectangular, being squared-off at the front and more rounded at the back (4) (6). While the upper shell (carapace) is yellow to dark brown, occasionally with a darker radiating pattern, the lower shell (plastron) is usually yellow, although this varies between the five subspecies (4) (6). The legs are covered in large scales, as is the long, thin, muscular tail, which is often used to support the animal’s weight like an additional limb (6) (7).

The big-headed turtle prefers fast-flowing, cool, rocky mountain brooks and streams, usually narrower than a metre wide and less than 10 centimetres deep (2) (4).

This turtle appears to be nocturnal, spending much of the day underwater burrowed into gravel deposits or hidden in rock crevices, generally at the stream edge or behind a waterfall (2) (4). At night, individuals emerge to search for food along the stream bottom, and sometimes also out of the water along the stream’s edge (4). Big-headed turtles are accomplished climbers, but not so well-equipped for swimming, with the ability to cling to rocks being more advantageous than swimming in the rapid waters in which it lives(2) (4). The diet is almost entirely carnivorous, with the species’ strong, bony jaws allowing it to feed on crustaceans and molluscs, which form a large portion of the diet (4).

The reproductive biology of big-headed turtles is almost completely unknown (6). A clutch reportedly contains one to six eggs, although two to three are more normal (2).

Legal and illegal trade are thought to pose the main threats to the big-headed turtle, which commonly appears in Asian food markets, and is in particular demand in China, Lao PDR and Vietnam (1) (6) (8). The species is also sold by pet traders and, sadly, its increasing scarcity in the wild is only likely to lead to a greater demand for the species and higher market value, encouraging further collection from the wild. Furthermore, since the reproductive rate is thought to be very low, populations of this species are believed to be very sensitive to removal of individuals and are slow to recover (8).

The big-headed turtle is legally protected in parts of its range and is likely to occur in protected areas (8). However, extensive and significant illegal international trade apparently continues. Captive breeding cannot supply demand, but does occur on an extremely limited scale in Thailand, and reportedly also occurs for commercial sale on Chinese turtle farms (8).

Crustaceans: diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.

Molluscs: a diverse group of invertebrates, mainly marine, that have one or all of the following; a horny, toothed ribbon in the mouth (the radula), a shell covering the upper surface of the body, and a mantle or mantle cavity with a type of gill. Includes snails, slugs, shellfish, octopuses and squid.

Nocturnal: active at night.

Plastron: the lower shell of a turtle or tortoise.

Subspecies: a population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.